1. Field of the Invention
The present invention relates to an apparatus and a method for producing sheets of glass. In particular, the sheets of glass have two faces, at least one which presents a high-quality surface.
2. Technical Background
A number of processes have been developed for fabricating glass sheets having highly smooth surfaces, such as can be used in, for example, computer display and television monitor applications. In one such method, the overflow downdraw fusion method, two flows of glass are generated by controlled overflow around a refractory isopipe. The two flows are kept in contact with the isopipe and are reunited at the root, or bottom tip, of the isopipe to form a sheet of semisolid glass. The two faces of the sheet of glass, thus, never come in contact with any surface that could damage or impart imperfections to the glass surface.
The speed of travel of the sheet of glass is determined by margin wheels and/or pulling rolls that act on the sheet at its margins. Such pulling rolls can help control movement of the glass sheet, apply tension to the glass sheet, and control the thickness of the glass sheet. Physical contact, for example, between the glass surface and a pulling roll, can damages the portion of the sheet in contact with the pulling roll, requiring subsequent removal of the margin portion of the glass sheets.
The fusion method is capable of producing sheets of glass presenting exceptional surface quality in terms of smoothness, thickness, and flatness or planarity. This method is commonly used to fabricate the sheets required for manufacturing flat screens based on light-emitting diodes (LEDs).
Nevertheless, the fusion method is not suitable for all types of glass compositions. For example, glass flow with the fusion method is controllable only if the flow of glass in contact with the isopipe, and more particularly only if the glass in contact with the bottom portion of the isopipe is maintained at a level of viscosity that is sufficiently high. If viscosity of the glass composition is lower than a threshold value, gravity forces can dominate over the viscosity forces and it can become difficult and/or impossible to appropriately tension the flow of semisolid glass leaving the root of the isopipe. In such a scenario, the flow of glass can become mechanically unstable and can generate a sheet of glass of poor quality having, for example variations in thickness and/or distortions. In view of mechanical stability, the fusion method conventionally requires a glass composition having a liquidus viscosity greater than about 10,000 pascal-seconds (Pa·s) (100,000 poises). Unfortunately, this raises a problem with glasses having a liquidus viscosity lower than this threshold. Such glasses, when in contact with the isopipe, can develop crystals at their interface with the isopipe, rendering glass sheets produced therefrom unsuitable for use in the demanding applications described herein.
Thus, there is a need to address the aforementioned problems and other shortcomings associated with the traditional fusion method for manufacture of glass sheets. These needs and other needs are satisfied by the methods of manufacture of the present invention.